1. Field of the Invention
The present invention relates to the field of optical receivers for free-space signal transmission. In particular, the present invention provides a novel optical receiver which utilizes an optical taper to achieve a high tolerance to angular misalignment, while supporting a high bit rate by permitting the use of a small surface area photodetector.
Optical tapers have been mainly used in integrated optical devices for the coupling of lightwaves between waveguides of different shapes and sizes. Known applications are concerned with the use of fiber tapers to improve the axial and transversal displacement tolerance and to reduce the dust sensitivity of single mode fibers in the case of single mode fiber coupling.
The present invention, however, relates to free space atmospheric optical links. A typical receiver for a free space/atmospheric optical link uses an objective lens or a mirror to collect and focus a signal beam on a photodetector, usually a photodiode. In order to achieve a high bit rate, one must use a photodetector with a very small active area (as small as 0.01 mm.sup.2 or smaller) because the diode capacitance is directly proportional to its active area. This makes the receiver alignment very difficult. For example, the angular misalignment of a receiver telescope with a focal length L=1 m and a photodetector with a lateral dimension d=0.1 mm must be smaller than d/L, namely 10.sup.-4 rad (about 20 second), which means that turbulence of atmosphere or even a small amount of jitter may cause complete loss of the signal beam.